By ELIZABETH VALK LONG President

When TIME decided to undertake this week's story about the beleaguered Bureau of Alcohol, Tobacco and Firearms, the editors knew immediately where to turn. Senior writer Erik Larson, who joined the staff in February, is a recognized authority on America's gun culture, having published a series of groundbreaking front-page stories on the subject for the Wall Street Journal and a critically acclaimed book, Lethal Passage: The Journey of a Gun (Crown; 1995).

Larson's interest in guns and gun control was aroused in the early 1980s by a series of teenage drive-by shootings that rocked the quiet neighborhood in Maryland where he lives with his wife, three young daughters and a goldfish named Joey.

"I set out to find where these kids were getting the guns," Larson recalls. "So I looked for a story that showed how weapons were diverted from good guys to bad guys." He found his answer in the chilling case of a troubled Virginia teenager, who on a cold December day in 1988 walked into his high school with a semiautomatic handgun and began shooting, killing one teacher and wounding another. That incident was the starting point for the two-year investigation that produced Lethal Passage.

Such dogged reporting is a hallmark of Larson's work. Raised in Freeport, Long Island, and a graduate of the University of Pennsylvania, Larson, 41, first became interested in journalism after seeing All the President's Men in 1978. "I thought, 'That looks like fun,'" he recalls. After graduating from the Columbia University Graduate School of Journalism, he pursued a variety of beats, including computers and technology, before turning to the gun trade. "Erik is meticulous and never gives up," says senior editor Priscilla Painton. "His stories are as solid as he is."

For this week's story, Larson felt the first question that needed answering was simply: What is the ATF? "We all knew the image conjured by the N.R.A. of jackbooted fascists raiding the homes of innocents," he says. What Larson found instead was a deeply divided and demoralized bureaucracy.

Getting ATF agents to open up about their lives and work required unusual tact and tenacity. "There was a great fear of retaliation by the ATF management," he says. After two months of travel, scores of interviews and a stack of documents piled knee high in his office, he delivered a penetrating look at an agency at war not with the militias but with itself.